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Cite this: Dalton Trans., 2014, 43,17324

Received 8th August 2014,Accepted 11th September 2014

DOI: 10.1039/c4dt02411g

www.rsc.org/dalton

Europium bis(dimethylsilyl)amides includingmixed-valent Eu3[N(SiHMe2)2]6[μ-N(SiHMe2)2]2†

André M. Bienfait,a Christoph Schädle,b Cäcilia Maichle-Mössmer,b Karl W. Törnroosa

and Reiner Anwander*b

Trivalent Eu[N(SiHMe2)2]3(THF)2 can easily be synthesized by applying a routine salt metathesis protocol

(EuCl3(THF)2 and 3 equiv. of Li[N(SiHMe2)2] in n-hexane) which crystallizes isotypically to its analogues of

the rare-earth metal series (space group P21/c). Transsilylamination of Eu[N(SiMe3)2]2(THF)2 with a slight

excess of HN(SiHMe2)2 in n-hexane–THF yields the divalent trinuclear compound Eu{[μ-N(SiHMe2)2]2Eu-

[N(SiHMe2)2](THF)}2, the solid-state structure of which differs significantly from the samarium and ytter-

bium analogues by showing three unique molecules in the asymmetric unit of which one is related to the

two others by an inversion. Using crude Eu[N(SiMe3)2]3 in transsilylamination reactions with HN(SiHMe2)2 in

n-hexane afforded n-hexane-insoluble trivalent ate complexes {MEu[N(SiHMe2)2]4}n (M = Na, K) depend-

ing on the synthesis conditions of Eu[N(SiMe3)2]3. Performing the transsilylamination of Eu[N(SiMe3)2]3with a large excess of HN(SiHMe2)2 at elevated temperatures gave reproducibly the donor-free, mixed-

valent, trinuclear compound EuII{[μ-N(SiHMe2)2]EuIII[N(SiHMe2)2]3}2 in good yield.

Introduction

Complexes Ln[N(SiHMe2)2]3(THF)x (Ln = rare-earth metal; x =1, 2) have emerged as versatile synthesis precursors accordingto the “extended silylamide route”,1–4 uniquely complement-ing Bradleys’ archetypal Ln[N(SiMe3)2]3 in terms of steric flexi-bility.5 Moreover, bis(dimethylsilyl)amide complexes havebeen frequently applied in surface organometallic chemistry(SOMC) due to the occurrence of mild surface-promotedamine elimination reactions and the presence of the Si–Hmoiety as a useful spectroscopic probe.6,7 We alsodescribed the synthesis of divalent Sm{[μ-N(SiHMe2)2]2Sm-[N(SiHMe2)2](THF)}2

8 and, more recently, tetravalent Ce-[N(SiHMe2)2]4

9 to be exploited as precursors in protonolysisreactions. Ever since the initial synthesis of the di- and tri-valent bis(dimethylsilyl)amide complexes, we have been intri-gued by the solvent-free variants “Ln[N(SiHMe2)2]x” (x = 2,3),which are anticipated to be superior precursors for the gene-ration of catalytically active organolanthanide complexes.Based on the fact that donor- and ate-complex-free trivalent

“Ln[N(SiHMe2)2]3” are not accessible by simple salt metathesisreactions, alternative protocols had to be explored. In 2006, weaccomplished the synthesis of {Y[N(SiHMe2)2]3}2 via an alkaneelimination reaction from polymeric [YMe3]n with three equiv.of HN(SiHMe2)2.

10 Since this synthesis is highly elaborate,transsilylamination reactions with homoleptic rare-earth metalbis(trimethylsilyl)amide complexes Ln[N(SiMe3)2]3 have beenenvisaged.5 Trans(silyl)amination reactions exploit the differentpKa values of (silyl)amines. In this particular case, HN-(SiHMe2)2 (pKa value 22.6) is used to displace the [N(SiMe3)2]

−

ligand, liberating HN(SiMe3)2 (pKa value 25.8).11,12 Ideally, thereleased silylamine can simply be removed – along with thesolvent – under reduced pressure, leaving the product inquantitative yields. The transsilylamination protocol proveditself useful for the synthesis of divalent Sm{[μ-N-(SiHMe2)2]2Sm[N(SiHMe2)2](THF)}2, which is not available viaa salt metathesis reaction either.8 In 2008, donor-free {La-[N(SiHMe2)2]3}2 and “Lu[N(SiHMe2)2]3” were found to be avail-able via transsilylamination conducted in n-hexane, even thoughthe lutetium compound was not isolated.13 When performedin the presence of a coordinating solvent like THF and at elev-ated temperatures, the respective reaction of Ln[N(SiMe3)2]3with HN(SiHMe2)2 provided rapid access to Ln[N(SiH-Me2)2]3(THF)y (Ln = Sc, Y, Sm, Nd, Lu).14 The transsilylamina-tion protocol has been further applied for the synthesis ofmain group15 and d-transition metal bis(dimethylsilyl)amidecomplexes.16

In this work, the transsilylamination protocol was probedfor the synthesis of europium bis(dimethylsilyl)amide

†CCDC 1018196–1018200 for 1–5. For crystallographic data in CIF or other elec-tronic format see DOI: 10.1039/c4dt02411g

aDepartment of Chemistry, University of Bergen, Allégaten 41, N-5007 Bergen,

NorwaybInstitut für Anorganische Chemie, Eberhard Karls Universität Tübingen, Auf der

Morgenstelle 18, D-72076 Tübingen, Germany.

E-mail: reiner.anwander@uni-tuebingen.de

17324 | Dalton Trans., 2014, 43, 17324–17332 This journal is © The Royal Society of Chemistry 2014

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complexes leading to the isolation of the donor-free, mixed-valent,trinuclear compound EuII{[μ-N(SiHMe2)2]Eu

III[N(SiHMe2)2]3}2.

Results and discussionSynthesis and solid-state structure of Eu[N(SiHMe2)2]3(THF)2 (1)

According to the original protocol,2 reaction of EuCl3(THF)2with 2.9 equiv. of Li[N(SiHMe2)2]

11 in n-hexane gave thecomplex Eu[N(SiHMe2)2]3(THF)2 (1) in high yield (93%) andhigh purity (Scheme 1). Complex 1 can easily be crystallizedfrom saturated n-hexane solutions at −35 °C forming deepyellow brick-like crystals (67%).

Single crystal X-ray crystallography showed that the euro-pium center is coordinated in a slightly distorted trigonalbipyramidal fashion (Fig. 1). Hence, the structure is isotypicwith the Y,1 La,2 Ce,17 Pr,17 Nd,1 Sm,18 Gd,19 Ho,20 Yb,21 andLu2 congeners. The nitrogen atoms of the three [N(SiHMe2)2]

−

ligands span the equatorial plane, while the sterically demand-ing SiHMe2 groups are twisted distinctly relative to the plane.

The SiHMe2 groups attached to the nitrogen closest tothe europium atom (N1) are most rotated out of the equatorialNNN plane as can be seen by the torsion angles (N3–Eu–N1–Si1, −49.64(7)°; N3–Eu–N1–Si2, 129.01(5)°), seemingly pushingthe two THF molecules out of the bipyramidal axis. The SiHMe2groups attached to N2 and N3, which are just slightly twisted,force the nitrogen atoms of the ligands into an angle >120°(132.17(3)°). The central europium atom is located slightly abovethe NNN plane towards one apical oxygen atom (O1).

Synthesis and solid-state structure of{Eu3[N(SiHMe2)2]2[μ-N(SiHMe2)2]4(THF)2} (2)

The synthesis of Eu{[μ-N(SiHMe2)2]2Eu[N(SiHMe2)2](THF)}2 (2)was performed in a manner similar to that of the analogoussamarium compound.8 The salt metathesis reaction ofEuI2(THF)2

22 with 1.9 equiv. of Na[N(SiMe3)2] at ambient temp-erature in THF gave Eu[N(SiMe3)2]2(THF)2

23 in high yield.Subsequent transsilylamination employing a small excess(2.4 equiv.) of HN(SiHMe2)2 afforded Eu{[μ-N(SiHMe2)2]2Eu[N-(SiHMe2)2](THF)}2 (2). Single crystals of 2 were obtained fromn-hexane solution at −35 °C forming small bright-yellow rhom-bohedra (Scheme 2).

X-ray structure analysis revealed a trinuclear complex com-parable with the samarium8 and ytterbium24 derivatives(Fig. 2, Table 1). Three distinct conformers in the asymmetricunit are unique to this europium derivative. Two of these (A,B) are oriented in the same direction while the third (C) isrelated to the former two by an inversion center. The foremostconformational differences are due to rotation of the silylgroups around the Si–N bond. The central europium atom(Euc) of each molecule is coordinated by four bridging nitro-gen atoms (Nb) from four [N(SiHMe2)2]

− ligands in a distortedtetrahedral fashion. The terminal europium atoms (Eut) arealso 4-coordinated by two Nb, one terminal nitrogen (Nt) froman additional [N(SiHMe2)2]

− ligand and one oxygen atom fromTHF. Conformers A and B differ from C, amongst other things,in the coordination around the Euc atom. The average Euc–Nb

distances for A and B are 2.613 and 2.616 Å, whereas theaverage Euc–Nb distance for C is 2.657 Å. The average Eut–Nb

distances for A (2.688 Å) and B (2.682 Å) are significantlylonger than the aforementioned Euc–Nb bridging ones. In con-trast, molecule C shows the rather “expected” average Eut–Nb

Scheme 2 Transsilylamination of Eu[N(SiMe3)2]2(THF)2 with HN(SiHMe2)2.

Scheme 1 Synthesis of 1 via salt metathesis of EuCl3(THF)2 withLi[N(SiHMe2)2].

Fig. 1 Molecular structure of Eu[N(SiHMe2)2]3(THF)2 (1). Heavy atomsare represented by atomic displacement ellipsoids at the 50% level;hydrogen atoms, except those attached to silicon, are omitted for clarity.Selected interatomic distances (Å) and angles (°): Eu–N1 2.2888(9),Eu–N2 2.3179(9), Eu–N3 2.3170(9), Eu–O1 2.4718(8), Eu–O2 2.4470(8),Eu⋯Si1 3.5488(3), Eu⋯Si2 3.2245(3), Eu⋯Si3 3.5094(3), Eu⋯Si4 3.3485(3),Eu⋯Si5 3.4676(3), Eu⋯Si6 3.4540(3); N1–Eu–N2 116.51(3), N1–Eu–N3110.79(3), N2–Eu–N3 132.17(3), O1–Eu–O2 163.02(3), Si1–N1–Si2127.64(6), Eu–N1–Si1 125.12(5), Eu–N1–Si2 107.23(4), Eu–N2–Si3120.71(5), Eu–N2–Si4 112.17(5), Eu–N3–Si5 118.46(4), Eu–N3–Si6 117.64(5).

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and Euc–Nb distances of 2.640 Å and 2.657 Å, respectively. Theaverage Euc⋯Eut distance of A and B is 3.664 and 3.687 Å, thatof C, however, is 3.817 Å. The angles Eut⋯Euc⋯Eut are ∼133°in A and B, and ∼150° in molecule C. All distances Eut⋯Nt

and Eut–O are comparable, averaging 2.448 and 2.556 Å,respectively. The agostic interactions, observed and intensivelydiscussed by Nagl et al.8 for the samarium analogue, can befound in the Eu structure as well. The DRIFT (Diffuse Reflec-tance Infrared Fourier Transform) spectrum of 2 illustratesthese secondary interactions with signals at ν (cm−1) = 2130(w), 2040 (m) and 1970(m).

Synthesis and solid-state structure of {MEu[N(SiHMe2)2]4}(M = Na (3), K (4))

In an attempt to generate donor-free {Eu[N(SiHMe2)2]3}, Eu[N-(SiMe3)2]3 was reacted with slightly more than 3 equiv. of HN-(SiHMe2)2 in n-hexane to perform a transsilylamination. When-ever crude, meaning not sublimed, Eu[N(SiMe3)2]3 was used,the formation of n-hexane-insoluble ate complexes {MEu[N-(SiHMe2)2]4} (M = Na (3), K (4)) was observed, depending on thesynthesis conditions of Eu[N(SiMe3)2]3. These side productswere characterized by elemental analysis, DRIFT, and single

Fig. 2 Upper part: the asymmetric unit of the crystal structure of Eu{[μ-N(SiHMe2)2]2Eu[N(SiHMe2)2](THF)}2 (2). Heavy atoms are representedby atomic displacement ellipsoids at the 50% level; hydrogen atoms, except those attached to silicon, are omitted for clarity. Lower part: the“backbone” of 2; torsion angles N1–Eu1–Eu3–N6 (−25.47°), N7–Eu4–Eu6–N12 (−25.54°) and N13–Eu7–Eu9–N18 (20.88°) are represented by redlines.

Paper Dalton Transactions

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crystal X-ray structure analysis. The solid-state structures of3 and 4 are isostructural to {KCe[N(SiHMe2)2]4}

25 featuring infi-nite chains (Fig. 3, Table 2). Slight differences are caused by thevarying ionic radii of Na+ and K+. Within the alternating Eu[N-(SiHMe2)2]4

– and M+ units, the Eu centers form a 157.71°/158.94° (3/4) angle with the alkaline metal centers, which them-selves are linearly arranged (3, 180.0(1)°; 4, 179.98°), leading to

an overall zig-zag geometry. The Eu–N distances of 3 and 4 are2.308(3)/2.338(3) Å and 2.312(3)/2.353(3) Å, respectively, andtherewith marginally longer than those in 1 (2.2888(9)–2.3179(9)Å). The alkaline metals appear sterically shielded in a distortedoctahedral fashion via agostic interactions with six Si–H moi-eties of the amido ligands (shortest distances: Na–H2 2.25(3) Åin 3, K–H3 2.67(3) Å in 4). The residual two Si–H moietiesinteract with the Eu center (shortest distances: Eu–H1 2.69(4)Å in 3, Eu–H1 2.73(5) Å in 4). Again, these secondary inter-actions can be detected in the IR spectrum of 4 at ν (cm−1) =2039, 2007. Even though different agostic interactions arepresent in both solid-state structures, they cannot distinctivelybe observed in the IR spectrum of 3, where only one broadsignal appears at ν (cm−1) = 2002. Dimeric {Li[Y[N(SiHMe2)2]4]}2is another related complex featuring similar intermolecularSiH → Li interactions in the solid state.26

Synthesis and solid-state structure of {Eu3[N(SiHMe2)2]6-[μ-N(SiHMe2)2]2} (5)

Given several unsuccessful attempts to obtain salt- and donor-free {Eu[N(SiHMe2)2]3}, we tried to force the envisaged transsil-

Table 1 Selected interatomic distances (Å) and angles (°) for molecules A, B and C of compound 2

Molecule A Molecule B Molecule C

DistancesEu1–N1 2.468(2) Eu4–N7 2.435(3) Eu7–N13 2.455(3)Eu1–O1 2.5510(19) Eu4–O3 2.567(2) Eu7–O5 2.559(2)Eu1–N2 2.697(2) Eu4–N8 2.707(2) Eu7–N14 2.604(2)Eu1–N3 2.678(2) Eu4–N9 2.646(2) Eu7–N15 2.656(2)Eu2–N2 2.593(2) Eu5–N8 2.609(2) Eu8–N14 2.663(2)Eu2–N3 2.635(2) Eu5–N9 2.622(2) Eu8–N15 2.634(2)Eu2–N4 2.644(2) Eu5–N10 2.622(2) Eu8–N16 2.668(2)Eu2–N5 2.579(2) Eu5–N11 2.610(2) Eu8–N17 2.662(2)Eu3–N4 2.655(2) Eu6–N10 2.664(2) Eu9–N16 2.637(2)Eu3–N5 2.720(2) Eu6–N11 2.709(2) Eu9–N17 2.661(2)Eu3–N6 2.444(2) Eu6–N12 2.449(2) Eu9–N18 2.438(2)Eu3–O2 2.5629(19) Eu6–O4 2.534(19) Eu9–O6 2.562(2)Eu1⋯Eu2 3.64262(18) Eu4⋯Eu5 3.7136(2) Eu7⋯Eu8 3.8128(2)Eu2⋯Eu3 3.6853(2) Eu5⋯Eu6 3.66023(19) Eu8⋯Eu9 3.8199(2)AnglesEu1–N2–Eu2 87.02(6) Eu4–N8–Eu5 88.60(6) Eu7–N14–Eu8 92.74(7)Eu2–N3–Eu1 86.56(6) Eu5–N9–Eu4 89.64(6) Eu8–N15–Eu7 92.22(7)Eu1–Eu2–Eu3 132.624(5) Eu4–Eu5–Eu6 133.362(5) Eu7–Eu8–Eu9 149.859(5)Eu1–N1–Si1 135.15(12) Eu4–N7–Si13 120.01(17) Eu7–N13–Si25 129.61(13)Eu1–N1–Si2 98.29(10) Eu4–N7–Si14 107.23(15) Eu7–N13–Si26 101.69(11)Eu1–N2–Si3 94.30(9) Eu4–N8–Si15 93.80(10) Eu7–N14–Si27 123.76(12)Eu1–N2–Si4 127.71(11) Eu4–N8–Si16 127.39(11) Eu7–N14–Si28 94.13(10)Eu1–N3–Si5 128.51(11) Eu4–N9–Si17 93.50(9) Eu7–N15–Si29 103.69(11)Eu1–N3–Si6 92.39(9) Eu4–N9–Si18 120.80(11) Eu7–N15–Si30 114.55(12)Eu2–N2–Si3 128.20(11) Eu5–N8–Si15 130.25(12) Eu8–N14–Si27 94.26(10)Eu2–N2–Si4 94.99(9) Eu5–N8–Si16 94.09(9) Eu8–N14–Si28 123.87(12)Eu2–N3–Si5 94.95(9) Eu5–N9–Si17 125.73(11) Eu8–N15–Si29 122.70(12)Eu2–N3–Si6 132.66(11) Eu5–N9–Si18 96.10(9) Eu8–N15–Si30 94.90(10)Eu2–N4–Si7 130.81(12) Eu5–N10–Si19 96.42(10) Eu8–N16–Si31 125.05(12)Eu2–N4–Si8 125.48(11) Eu5–N10–Si20 129.06(12) Eu8–N16–Si32 93.47(10)Eu2–N5–Si9 129.59(11) Eu5–N11–Si21 94.12(9) Eu8–N17–Si33 92.56(10)Eu2–N5–Si10 95.14(9) Eu5–N11–Si22 133.47(11) Eu8–N17–Si34 134.46(12)Eu3–N4–Si7 92.22(9) Eu6–N10–Si19 123.43(11) Eu9–N16–Si31 93.67(10)Eu3–N4–Si8 125.48(11) Eu6–N10–Si20 92.89(9) Eu9–N16–Si32 124.15(11)Eu3–N5–Si9 93.74(9) Eu6–N11–Si21 123.12(11) Eu9–N17–Si33 119.37(11)Eu3–N5–Si10 127.84(11) Eu6–N11–Si22 92.88(9) Eu9–N17–Si34 93.52(10)Eu3–N6–Si11 111.36(11) Eu6–N12–Si23 127.92(13) Eu9–N18–Si35 111.87(12)Eu3–N6–Si12 117.85(12) Eu6–N12–Si24 102.38(12) Eu9–N18–Si36 121.94(12)

Fig. 3 Cutout of the polymeric chain of {NaEu[N(SiHMe2)2]4} (3). Heavyatoms are represented by atomic displacement ellipsoids at the 50%level; hydrogen atoms, except those attached to silicon, are omitted forclarity.

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ylamination by increasing the concentration of HN(SiHMe2)2and the reaction temperature at the same time. When atranssilylamination reaction of Eu[N(SiMe3)2]3 (which should besublimed for maximum yield and purity) with a 10-fold excessof HN(SiHMe2)2 (according to the amount of ligands) was per-formed at 60 °C in n-hexane, the mixed-valent compoundEuII{[μ-N(SiHMe2)2]Eu

III[N(SiHMe2)2]3}2 (5) could be reproduci-bly isolated in good yields (Scheme 3). In later experiments itwas found that neither the elevated temperature nor theextended reaction time (7 d) influenced the outcome. The sameproduct can be obtained by a reaction at ambient temperatureand a reaction time of 18 h. Compound 5 was characterized byelemental analysis, DRIFT spectroscopy, and single crystal X-raystructure analysis.

It seems that the majority of mixed-valent rare-earth metalcomplexes arise from (at times hard-to-control) oxidations. Forexample, treatment of rare-earth metals Eu0, Sm0, andYb0 with a 3,5-di-tert-butylpyrazole (tBu2pzH) and mercuryat 220 °C afforded the mixed-valent ytterbium complex[(η2-tBu2pz)YbII(µ–η2:η2-tBu2pz)2YbIII(η2-tBu2pz)2], whilst thesame reaction with europium and samarium gave divalent andtrivalent compounds, respectively.27 In a similar reaction

employing rare-earth metals and a carbazole (CbzH) in a melt,samarium formed a polymeric mixed-valent chain of{[Sm2(Cbz)5](CbzH)}-subunits, whilst europium and ytterbiumboth formed divalent complexes.28 The reaction of metallicytterbium, (C6H5)Hg(C6F5), and HN(SiMe3)2 in THF affordedthe mixed-valent compound [YbII(C6F5)(THF)5][Yb

III(C6F5)2-{N(SiMe3)2}2] in low yield.29 Divalent Yb[N(SiMe3)2]2(L)2(L = THF or Et2O) reacts with a cross-bridged cyclam ligand(H2CBC) to small amounts of {[YbIII(CBC)]3[µ3-NH]}+{YbII-[N(SiMe3)2]3}

−.30 Redox reaction of divalent [Yb(C9H7)2(THF)2]in either a 1 : 2 or 1 : 1 molar ratio with a diazabutadiene givesdinuclear [Yb2(µ–η5:η4-C9H7)(η5-C9H7)2{µ–η4:η4-PhNC(Me)vC(Me)NPh}] or tetranuclear [Yb2(µ–η5:η4-C9H7)(η5-C9H7)2{µ–η4:η4-PhNC(CH2)vC(Me)NPh}]2, respectively.31 Di-valent Ln[N(SiMe3)2]2(thf)2 (Ln = Sm, Eu, Yb) and a tridentateamine bis(phenol) reacted to produce another example ofmixed-valency (SmIII

2SmIIL4), while europium and ytterbium

stayed in their divalent oxidation state.32 Reacting metallicytterbium with PhI in 1,2-dimethoxyethane (dme) gave “PhYbI-(dme)n”, from which ion-separated [{YbII(dme)4}-{YbIIIPh4(dme)}2] arose in a fractional crystallization fromdme/n-hexane.33 Even though the vast majority of mixed-valentcompounds stems from oxidations, spontaneous reductionsalso can lead to such species as revealed by the reaction ofytterbium tris(iodide) with 3 equiv. of [KBz] generating[YbII(Bz)(THF)5]

+[YbIII(Bz)4(THF)2]− and the [Bz]-oxidative

coupling product.34 In contrast, SmI3 is not reduced underthese conditions.34 Unfortunately, this reaction was not per-formed on europium, which likely would have been reducedentirely to the divalent oxidation state. We are tempted tospeculate that the reduction observed in our study might becaused by the Si–H moiety of the (pro-)ligand HN(SiHMe2)2.Compounds bearing Si–H functionalities like Et3SiH are wellknown reducing agents being mostly used in organic synth-eses,35 but are also known for their ability to reduce a numberof transition metals.36

The molecular structure of 5 features three Eu atoms,showing a bent arrangement (Eu2–Eu1–Eu2′ = 167.973(7)°,Fig. 4). Each of the terminal EuIII-atoms is coordinated in adistorted tetrahedral fashion by three terminal [N(SiHMe2)2]

−

ligands (Nt) and a bridging one (Nb). The metal nitrogenbonds of the terminal ligands are of similar lengths (2.281(3)–2.315(3) Å) and comparable to the europium nitrogen bonds incomplex 1 (2.2888(9)–2.3179(9) Å). The corresponding bonds ofthe anionic {Eu[N(SiHMe2)2]4}

− subunits of 3 (2.308(3)/2.337(3)Å) and 4 (2.312(3)/2.353(3) Å) are, due to the negative charge,somewhat longer. The Eut–Nb bonds in 5 are, as expected, sig-nificantly longer (2.483(3) Å) and compare with the EuII–Nt

bonds in complex 2 (∼2.468(2) Å). Again, the Euc–Nb distancesin 5 are significantly longer (2.699(3) Å) than those betweenEuIII and Nb, which was also expected because of the larger ionradius of EuII. Surprisingly, this bond is even longer than theaverage EuII–Nb bond in 2. This finding could not be expectedsince the central EuII atom of complex 5 is only coordinated bythe two Nb and is therefore formally still partially charged (+1).However, Euc is additionally stabilized by six SiH → Eu second-

Table 2 Selected interatomic distances (Å) and angles (°) for com-pounds 3 and 4

3 (Na) 4 (K)

DistancesEu–N1 2.338(3) 2.353(3)Eu–N2 2.308(3) 2.312(3)Eu⋯Si1 3.1689(9) 3.2052(11)Eu⋯Si2 3.6699(10) 3.6764(11)Eu⋯Si3 3.4226(9) 3.4543(11)Eu⋯Si4 3.3838(10) 3.3630(12)Eu⋯M 4.4642(3) 4.5919(4)M⋯Si2 3.3704(10) 3.6881(11)AnglesN1–Eu–N1′ 103.04(14) 107.72(16)N2–Eu–N2′ 99.01(13) 101.75(17)N1–Eu–N2 115.93(10) 117.32(11)N1–Eu–N2′ 111.76(10) 106.57(12)Eu–N1–Si1 102.33(14) 103.48(16)Eu–N1–Si2 130.69(15) 129.59(17)Eu–N2–Si3 116.86(15) 118.57(17)Eu–N2–Si4 114.95(14) 113.45(16)

Scheme 3 Transsilylamination of Eu[N(SiMe3)2]3 with excess ofHN(SiHMe2)2 at elevated temperature.

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ary interactions involving two bridging and four terminal silyl-amido ligands. The two nitrogen atoms and six SiH bondsform a distorted square antiprismatic geometry around theEuII centre. The agostic interactions can be observed in theDRIFT spectrum of 5, in which three separated Si–H vibrationsare present at ν (cm−1) = 2066 (with a shoulder at 2131), 1998,and 1934. Since, as aforementioned, the EuII is coordinated byonly two bridging ligands, it has theoretically a partial chargeof +1, whereas both EuIII atoms carry a partial charge of −0.5each, making the molecule zwitterionic-like. Hence, the struc-ture of 5 shows a bridging motif of the [N(SiHMe2)2]

− ligandentirely distinct from divalent 2 (central atom interacts with allfour nitrogen atoms of the bridging amido ligands) and atecomplex {Li[Y[N(SiHMe2)2]4]}2 (each lithium atom exhibits twoLi–N contacts to amido ligands of the same yttrium centre andintermolecular SiH → Li interactions). This electronic featureand the low coordinative saturation of the central europiumatom might provide an interesting reactivity behaviour.For example, exposing 5 to donor molecules might resultin charge-separated species similar to [{YbII(dme)4}-{YbIIIPh4(dme)}2].

33

Conclusions

The formation of ancillary ligand-free europium bis(dimethyl-silyl)amide complexes is crucially affected by the synthesisroute. While the routine salt metathesis of EuCl3(THF)2 withthree equivalents of Li[N(SiHMe2)2] performed in n-hexaneresulted in complex EuIII[N(SiHMe2)2]3(THF)2, a transsilyl-amination of Eu[N(SiMe3)2]3 using a large excess of HN(SiHMe2)2in n-hexane, gave the mixed-valent “zwitterion”-like compound

EuII{[μ-N(SiHMe2)2]EuIII[N(SiHMe2)2]3}2 in high yield and

purity. Further transsilylamination reactions showed that (a)EuII[N(SiMe3)2]2(THF)2 gave virtually quantitative conversioninto the trinuclear divalent compound EuII{[μ-N(SiH-Me2)2]2Eu

II[N(SiHMe2)2](THF)}2, and (b) the presence of alkalimetal ions may stabilize the trivalent state as shown for theisolation of MEuIII[N(SiHMe2)2]4 (M = Na, K). At this point, onecan only speculate about the ability of HN(SiHMe2)2 to act asa reductant in transsilylamination reactions. X-ray crystallo-graphy revealed that (a) the donor-free europium centers inEuII{[μ-N(SiHMe2)2]Eu

III[N(SiHMe2)2]3}2 and EuII{[μ-N(SiH-Me2)2]2Eu

II[N(SiHMe2)2](THF)}2 feature extensive SiH → Eusecondary interactions and (b) the latter divalent complex doesexist as distinct conformers in the solid state (in contrast tothe samarium and ytterbium congeners).

ExperimentalGeneral considerations

All manipulations were performed under rigorous exclusion ofair and moisture, using glovebox techniques (MB BraunMB150B-G-II; <1 ppm O2, <1 ppm H2O, argon atmosphere)and pressure tubes. The solvents diethyl ether, n-hexane, tetra-hydrofurane (THF) and toluene were purified using Grubbscolumns (MBraun SPS, solvent purification system). 1,1,3,3-Tetramethyldisilazane (97%) and 1,1,1,3,3,3-hexamethyldisil-azane (95%) purchased from ABCR were used as received,1,2-diiodoethane (99%) purchased from Sigma-Aldrich wasrecrystallized from diethyl ether prior to use, europium metal(99.9%) purchased from Sigma-Aldrich was used as received,europium(III) chloride (99.9%) purchased from ABCR was usedas received or transferred into its THF-adduct, EuCl3(THF)2, bySoxhlet-extraction, n-butyl lithium (1.6 M in hexanes) and pot-assium bis(trimethylsilyl)amide (95%) were purchased fromSigma-Aldrich and used as received, sodium bis(trimethylsilyl)-amide (95%) was purchased from FLUKA and used as received,or synthesized by reacting sodium amide (95%) purchasedfrom Sigma-Aldrich with 1.1 eq. of 1,1,1,3,3,3-hexamethyldisil-azane in hexane. Li[N(SiHMe2)2]

10 and Eu[N(SiMe3)2]2(THF)237

were synthesized according to literature procedures. IR spectrawere recorded on a Thermo Scientific Nicolet 6700 FT-IR andon a Nicolet protégé 460 using DRIFT (Diffuse ReflectanceInfrared Fourier Transform). For all the DRIFT measurements,the ratio of potassium bromide to metal complex was kept con-stant at 20 : 1. Elemental analyses were performed on an Ele-mentar Vario MICRO cube.

Synthesis of Eu[N(SiHMe2)2]3(THF)2 (1). EuCl3(THF)2(0.500 g, 1.24 mmol) was stirred with Li[N(SiHMe2)2] (0.502 g,3.60 mmol) for 18 h at ambient temperature in 18 ml n-hexanegenerating a yellow solution. All solids were removed by cen-trifugation, the yellow n-hexane solution was filtered and thesolvent was removed under reduced pressure. The yellowresidue was dissolved in n-hexane, filtered, and dried again,leaving analytically pure 1 (0.794 g, 1.15 mmol, 93%). Com-pound 1 crystallized from a concentrated n-hexane solution at

Fig. 4 Molecular structure of EuII{[μ-N(SiHMe2)2]EuIII[N(SiHMe2)2]3}2 (5).

Heavy atoms are represented by atomic displacement ellipsoids at the50% level; hydrogen atoms, except those attached to silicon, areomitted for clarity. Selected interatomic distances (Å) and angles (°).Eu1–N1 2.699(3), Eu2–N1 2.483(3), Eu2–N2 2.281(3), Eu2–N3 2.298(3),Eu2–N4 2.315(3), Eu1⋯Eu2 4.2821(5), Eu1⋯Si1 3.2086(10), Eu1⋯Si23.6486(10), Eu2⋯Si1 3.6782(10), Eu2⋯Si2 3.1902(10), Eu2⋯Si3 3.5507(12),Eu2⋯Si4 3.5507(12), Eu2⋯Si5 3.3502(11), Eu2⋯Si6 3.4583(11), Eu2⋯Si73.1493(11), Eu2⋯Si8 3.6338(10); Eu2–Eu1–Eu2’ 167.972(7), N1–Eu1–N1’130.02(11), Eu1–N1–Eu2 111.39(9), Eu1–N1–Si1 90.64(11), Eu1–N1–Si2109.22(13), Eu2–N1–Si1 121.73(14), Eu2–N1–Si2 97.01(12), Eu2–N2–Si3109.18(15), Eu2–N2–Si4 125.41(17), Eu2–N3–Si5 112.83(16), Eu2–N3–Si6120.03(16), Eu2–N4–Si7 102.24(14), Eu2–N4–Si8 130.38(16).

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−35 °C (two crops, 0.575 g, 0.83 mmol, 67%). DRIFT: ν (cm−1)= 2960 (s), 2900 (m), 2070 (s) shoulder at 1940, 1790 (vw), 1650(vw), 1440 (vw), 1250 (s), 1180 (vw), 1050 (s), 1030 (s), 895 (vs),835 (s), 789 (s), 762 (s), 687 (m), 625 (w), 604 (w). Anal. Calc.for C20H58EuN3Si6: C, 34.65; H, 8.43; N, 6.06%. Found: C,34.46; H, 8.40; N, 6.03%.

Synthesis of Eu{[μ-N(SiHMe2)2]2Eu[N(SiHMe2)2](THF)}2(2). The synthesis of 2 was carried out in a slightly modifiedmanner to that of the samarium analogue.8 A n-hexane (4 ml)solution of HN(SiHMe2)2 (0.213 g, 1.60 mmol) was slowlyadded to Eu[N(SiMe3)2]2(THF)2 (0.411 g, 0.67 mmol) dissolvedin a mixture of 6 ml n-hexane and 1 ml THF. During theaddition the colour of the solution changed from dark yellowto bright yellow. The reaction mixture was stirred for 18 h,after which the solvent was removed under vacuum leaving 2as a bright yellow solid (0.331 g). The product was dissolved ina 4 : 1 mixture of n-hexane–thf and filtered and the solutionwas concentrated under reduced pressure. Storage of the con-centrated solution at −40 °C yielded 2 as bright yellow crystals(0.226 g, 0.16 mmol, 72%). X-ray quality crystals were grownfrom a more diluted n-hexane solution at −35 °C. DRIFT: ν(cm−1) = 2950 (m), 2900 (w), step-like 2130 (w), 2040 (m), 1970(m), 1780 (vw), 1430 (vw), 1250 (s), 1180 (vw), 1070 (s), 1030 (s),985 (s), 930 (vs), 895 (vs), 839 (s), 789 (m), 764 (s), 708 (w),681 (w), 631 (vw), 592 (vw), 563 (vw). Anal. Calc. forC32H100Eu3N6Si12: C, 27.57; H, 7.23; N, 6.03%. Found: C,27.41; H, 7.01; N, 6.13%.

Synthesis of {NaEu[N(SiHMe2)2]4} (3). When attempting atranssilylamination reaction using unsublimed Eu[N(SiMe3)2]3,synthesized from EuCl3 and 3 equiv. of Na[N(SiMe3)2], smallamounts of compound 3 precipitated instantly from then-hexane solution during the addition of HN(SiHMe2)2. After18 h stirring at ambient temperature, 3 was separated by cen-trifugation from the solution, washed three times with smallamounts of n-hexane, and dried under vacuum. Due to its

solubility in toluene, X-ray suitable crystals of compound 3could be crystallized at −35 °C. DRIFT: ν (cm−1) = 2950 (m),2898 (w), 2002 (s, broad), 1415 (vw), 1251 (s), 1058 (s), 1042(vs), 942 (vs), 884 (vs), 835 (vs), 782 (s), 764 (s), 708 (w), 683 (w),626 (w), 596 (w), 404 (w). Anal. Calc. for C16H56EuNaN4Si8: C,27.29; H, 8.01; N, 7.96%. Found: C, 27.34; H, 8.27; N, 7.80%.

Synthesis of {KEu[N(SiHMe2)2]4} (4). Complex 4 wasobtained in a similar manner as 3 with the exception of usingK[N(SiMe3)2] for the synthesis of Eu[N(SiMe3)2]3. DRIFT:ν (cm−1) = 2948 (m), 2894 (w), 2039 (s), 2007 (vs), 1416 (vw),1247 (vs), 1065 (vs), 1041 (vs), 938 (vs), 884 (vs), 835 (vs), 779 (s),763 (s), 709 (w), 681 (w), 627 (w), 594 (w), 404 (w). Anal. Calc. forC16H56EuKN4Si8: C, 26.68; H, 7.84; N, 7.78%. Found: C, 26.06;H, 7.43; N, 7.39%. Crystals, suitable for single crystal X-ray ana-lysis, were grown by placing a small vial with a toluene solutionof “KEu[N(SiMe3)2]4-contaminated” Eu[N(SiMe3)2]3 into a largervial filled with HN(SiHMe2). The larger vial was closed tightlyand stored at ambient temperature. Already after 24 h, X-raysuitable single crystals were grown as colourless needles.

Synthesis of EuII{[μ-N(SiHMe2)2]EuIII[N(SiHMe2)2]3}2 (5). Eu-

[N(SiMe3)2]3 was synthesized by a slightly modified literatureprocedure.1,5 EuCl3(THF)2 (0.392 g, 0.97 mmol) was allowed toreact for 18 h with 2.95 equiv. of Na[N(SiMe3)2] (0.527 g,2.87 mmol) in 15 ml THF at ambient temperature. The productwas dried under vacuum and the solid residue was extractedwith n-hexane twice. The solution was filtered and dried undervacuum again. The orange crystalline product was sublimed at87 °C and 1.5 × 10−5 mbar. Anal. Calc. for C18H54EuN3Si6: C,34.15; H, 8.60; N, 6.64%. Found: C, 34.09; H, 8.72; N, 6.44%.

Initial synthesis: Eu[N(SiMe3)2]3 (0.100 g, 0.16 mmol) wasdissolved in 10 ml hexane and an excess of 30 equiv. HN(SiHMe2)(0.632 g, 4.74 mmol) was added to the solution. The mixture wasstirred for 7 d at 60 °C in a pressure tube. The pressure tube wastransferred into a glovebox, the solution was filtered, and all vola-tiles were removed under vacuum. A few X-ray quality crystals of 5

Table 3 Crystallographic data for complexes 1–5

1 2 3 4 5

Formula C20H58EuN3O2Si6 C32H100Eu3N6O2Si12 C16H56EuN4NaSi8 C16H56EuKN4Si8 C32H112Eu3N8Si16Fw 693.19 1394.14 704.31 720.42 1514.62T/K 123(2) 103(2) 173(2) 173(2) 173(2)Crystal system Monoclinic Monoclinic Monoclinic Monoclinic MonoclinicSpace group P21/c P21/n C2/c C2/c C2/ca/Å 13.0596(3) 15.3672(5) 22.5027(14) 19.5944(16) 10.8648(12)b/Å 16.3732(4) 39.2563(12) 11.2473(9) 11.0176(7) 20.1124(14)c/Å 16.6147(4) 32.0718(10) 17.5201(11) 18.0581(16) 33.327(3)β/° 90.82 90.434(1) 124.589(4) 106.222(7) 94.456(8)Z 4 12 4 4 4V/Å3 3552.31(15) 19347.1(11) 3651.1(5) 3743.2(5) 7260.5(12)ρcalcd/g cm−3 1.296 1.436 1.281 1.278 1.386µ/mm−1 1.987 3.134 2.004 2.055 2.851F(000) 1448 8484 1464 1496 3092Θ/° 1.75 to 32.10 1.37 to 32.10 4.14 to 26.37 4.18 to 25.34 3.14 to 29.16Reflections collected 66 356 36 9851 26 317 23 798 47 258Independentreflections

12 431 [R(int) =0.0150]

67 610 [R(int) =0.0302]

3722 [R(int) =0.1287]

3418 [R(int) =0.1316]

9681 [R(int) =0.0930]

Goodness-of-fit on F2 1.100 1.217 1.082 1.104 1.094R[I > 2σ(I)] 0.0144 0.0345 0.0318 0.0343 0.0359wR[I > 2σ(I)] 0.0380 0.0585 0.0667 0.0795 0.0720

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were received from n-hexane at −35 °C in the form of orangeblocks and used for the X-ray structure determination.

Improved synthesis: Eu[N(SiMe3)2]3 (0.393 g, 0.62 mmol)was dissolved in 10 ml n-hexane and an excess of 30 equiv.HN(SiHMe2) (2.484 g, 18.63 mmol) was added to the solution.The mixture was stirred for 18 h at ambient temperature. Thesolution was filtered and all volatiles were removed undervacuum. The product was crystallized from n-hexane at −35 °C inthe form of orange blocks (0.267 g, 0.18 mmol, 85%). X-ray analy-sis confirmed the unit cell of compound 5. DRIFT: ν (cm−1) =2952 (m), 2898 (w), 2066 (m) with a shoulder at 2131 (w), 1998(m), 1934(m), 1412 (vw), 1250 (s), 1167 (vw), 1028 (s), 959 (s),888 (vs), 835 (s), 786 (s), 764 (s), 704 (w), 685 (w), 626 (w),599 (w), 572 (vw). Anal. Calc. for C32H112Eu3N8Si16: C, 25.38; H,7.45; N, 7.40%. Found: C, 25.61; H, 7.06; N, 7.38%.

X-ray crystallography and crystal structure determination ofcomplexes 1–5

Crystals were grown by standard techniques from saturatedsolutions using n-hexane (1, 5) or toluene (3) at −35 °C; 2 wascrystallized from n-hexane–THF 4 : 1 at −40 °C. Due to the in-solubility of complex 4, crystals had to be grown by diffusion ofthe reactant into a toluene solution of the precursor at ambienttemperature. Suitable crystals for diffraction experiments wereselected in a glovebox and mounted in Paratone-N (HamptonResearch) on a nylon loop. Data collection for 1 and 2 was doneon a Bruker AXS TXS rotating anode APEXII CCD, and 3–5 on aSTOE IPDS 2 T using MoKα radiation (λ = 0.71073 Å). Structuresolution and final model refinement were done using Stoe’sX-Area,38 SMART39 and SAINT,39 WinGX suite,40 SHELXS9741

and SHELXL-97.41 All plots were generated using the programORTEP-3.42 Further details of the refinement and crystallo-graphic data are listed in Table 3 and in the CIF files.

Acknowledgements

We thank the Norwegian Research Council (Project No.182547/I30) and Meltzer-foundation at the University ofBergen.

Notes and references

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